This page contains additional information on the mbed.org-project. The mbed consists of an ARM Cortex M3 MCU that connects to your PC as USB mass storage device. To upload the code, you just copy the hex-File to that USB-drive and reset your MCU. That's it!
A main part of the mbed-project is the online compiler with its easy-to-use libraries and the code-sharing feature. While I recommend using the online-compiler for reasons of simplicity, you might encounter a point where you need to run pure ARM-C code on your mbed. Here's an explanation how to do that.
Installation of the ARM-ELF-GCC
sudo port install arm-elf-gcc
After that you have a version of the ARM-GCC installed in /opt/local/arm